The chamber Tuesday is scheduled to take an initial vote on a $21 billion bill from Bernie Sanders, I-Vt., that would reverse the last traces of the December cut, and then go further to expand veterans' education and health care benefits.

Those cuts—passed as part of the 2013 year-end bipartisan budget deal—would have slashed more than $6 billion in spending by lowering the cost-of-living benefits to medically retired soldiers. But despite the initial support, the cuts sparked a firestorm among veterans groups, and Congress has been scrambling to undo them ever since.

Earlier this month, the Senate voted to restore the cost-of-living benefits to military personnel that have already retired. Sanders' bill would restore the cuts to those who entered the military this year.

But though many members have proclaimed their dedication to reversing the cuts, the bid to do so may soon stall.

And though a sufficient number of Republicans are expected to get on board for Tuesday's vote to move the measure a small step forward bill, it's unclear whether that support will continue on future, more substantive votes to pass the measure.

Republicans are insisting on offering amendments of their own to Sanders' bill, including a plan to replace the measure with smaller alternative from Sen. Richard Burr, the Veterans Affairs panel's top Republican.

The GOP plan would also strike down Sanders' bill's method to pay for the expanded spending, which relies on saving from the drawdown of the wars in Iraq and Afghanistan. Republicans say those savings are "false," arguing they don't actually save taxpayers any money. Instead, Republicans want to pay for it by targeting a child tax credit used by undocumented immigrants, part of a previous measure from New Hampshire Republican Kelly Ayotte.

If Republicans don't get to vote on their amendments and if its spending offset isn't changed, they're threatening to block the measure—even if it that position leaves them at odds with most veterans groups.

"It would be very difficult for people to vote against a veterans bill," said Sen. Jim Inhofe, R-Okla. "But if they do it with the [war drawdown funding] offset there might be some of us who vote against it, and I might be one of them."

Sen. John McCain, R-Ariz., said it is a slap that Republicans are being blocked from amending such a massive bill for such a vital group.

"I think I know as much about veterans as Mr. Sanders, with all do respect, yet I'm not allowed a single amendment to Mr. Sanders' bill. That to me is an outrage and an insult," he said.

McCain accused Reid of trying to make Republicans look bad by playing partisan politics and likely putting them in a position to ultimately oppose the bill.

"He's just trying to embarrass us," McCain said. "It's just a tactic."

McCain said besides his opposition to Sanders pay-for, he also did not believe the bill addressed the most critical issues such as reforming the Veterans Affairs Department "which is not providing the services to veterans that they deserve," he said.

"You talk to any group of veterans, they'll tell you their No. 1 priority is, not anything in Sen. Sanders bill, but the service and the ability of them to get the benefits that they have already earned without this bill."

The debate over the Sanders measure comes as Senate Democrats have said they plan to take up a spate of issues viewed as possible election-year buoys for vulnerable incumbents, setting up what some Republicans view as an emerging trend whereby Reid moves favorable Democratic legislation to the floor, Republicans cry out for amendments, Reid fails to meet their demands, and the GOP ultimately blocks the measure. This pattern played out over an extension of unemployment-insurance benefits earlier this year.

"We should be allowed to try and amend bills," said Sen. Ron Johnson, R-Wis. "That's the way the Senate should work. It hasn't been working that way."

Republican Sen. Mike Johanns of Nebraska, who sits on the Veterans Affairs Committee, said he is likely to vote to proceed to the bill, but suspects Reid won't allow Republicans amendments.

"We'll just see if Senator Reid allows us to offer any amendments," he said. "That'll tell us whether he's serious about this bill or not. I must admit I'm suspicious that is gonna be just another show vote as we head toward November, but we'll see."